Serial killer Timothy Wilson Spencer was the first US man to be sentenced to death from DNA evidence. David Vasquez, wrongly convicted of one of Spencer's crimes, was the first man in the USA to be cleared by DNA testing.

In 1912, four-year-old Bobby Dunbar disappeared during a fishing trip but was found eight months later. Bobby was returned to his family, but another woman claimed he was her son. In 2004, DNA testing revealed the boy was not Bobby Dunbar - whose fate remains unknown.

Parental, forensic and genetic testing look for similarities in the genetic markers between two biological samples.

Because all cells in the body contain exactly the same DNA, samples can be taken from almost anywhere in the body, including skin, hair follicles, blood and other bodily fluids.

A forensic scientist might be asked to compare DNA from skin cells found underneath the fingernails of an attack victim, with the DNA from a blood sample taken from a potential suspect.

First of all, the DNA is isolated from the cells and millions of copies are made, using a method called 'polymerase chain reaction', or PCR.

PCR uses a naturally occurring enzyme to copy a specific stretch of DNA over and over again. Having lots of DNA makes the genetic code easier to analyse.

The DNA molecules are then split at particular locations to separate them into known 'chunks' and the code at those specific points is analysed to create a DNA fingerprint. The fingerprints from the two different samples are then compared to see if they match.

How accurate are DNA tests?

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Steve Jones explains the power of DNA testing

The accuracy of DNA tests has big implications. DNA tests are sometimes the only evidence to prove that a suspect was involved in a crime, or free someone who has been wrongly convicted.

It is easy to tell if DNA from two biological samples does not match. But a match doesn't make you totally certain that the two samples come from the same person.

There is always a small chance that two different people's genetic markers could be the same, especially if they are related.

To reduce the chance of error, scientists test more than one genetic marker. The more identical markers there are in two samples, the more accurate the test.

However, testing more markers takes more time and is more expensive. Forensic DNA tests usually examine six to ten markers. The chances that two unrelated people have identical profiles is less than one in one billion.

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